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Changing from a 5.7 Liter OMC to a 7.4 454 Mercruiser

03-02-2018, 03:05 PM

Hi ALL!!!

Well winter has passed and unfortunately I was one of the unfortunate few who had an engine that didnt survive. Our 1987 Bayliner Avanti 3255 port side engine has a cracked block and I am looking to replace it. Luckily for me I have found two 7.4 Mercruiser 454's with extremely low hours on them that are completely built up (minus exhaust manifolds and risers) for an unbelievable price and 12 month warranty. Plan is to replace both engines in my boat with these but I am of course out of my depth here in knowing whether or not these mercruiser's will actually match up to the OMC out drives that are currently installed.

Our local Mechanic unfortunately is out of town until next week and I dont want to miss out on these engines if indeed they will fit/match up to our current set up.

I repowered a 1987 2550 with a merc 454 with bravo I drive, even if they could match up, which I don't think they would, your omc drives would need to be king cobra drives. And I want to say your boat has cobra drives.

Comment

Since you are installing Mercruiser engines may as well go with Mercruiser Outdrives. I don't think that the OMC's will mate up to the Mercruisers. Golden opportunity here. I would recommend that you go with Bravo II drives at a minimum Bravo III's IMO would be even better.

Comment

The bolt pattern at the flywheel cover between the 7.4 (BBC) and the 5.7 (SBC) is the same, so technically if a 5.7 is bolted up the 7.4 will also bolt up. However as previously mentioned the drives may not do well with the additional torque of the bbc.

I probably wouldn't go 454 these days for a couple reasons. First is there.will be no space in the bilge. Second the weight difference will have a negative effect on your boat. Then there is the expense.of parts swapping. How about engine heigh will they be too tall for your hatch covers.

I would go 383 or similar if you are looking for power. There are a lot of considerations beyond just a bolt up.

check out the above links they are a write up on the avanti the next size up from yours. I can tell you that the factory 454 carbs already touch the hatch covers in mine. There is an imprint in the hatch cover insulation where it touches so check your carb to hatch clearance. With 454s you will have no clearance between the engines and changing spark plugs is going to be a real pain in the butt. If you like i can take some measurements from mine. I have a 32 ft with 350's and a 3450 with 454s. As of now my 32 doesn't have a lot of exhaust to exhaust space between the engines.

I'm not trying to be negative but realistic. I'm always one for more power. If I thought I thought it would work I would do it but I think you can get just as much power 383 it with much less weight and better fuel economy. However i do understand the fact that you found twins that you.possibly can use. Just make sure they are going to fit.

Comment

Yatchman makes good point about loosing space and adding weight. 383's should be an awsome choice if you have to go with new power. Have you thought about a possible JB weld repair on the crack? I've never done this myself but others have saved engines doing this, might be worth a try if your engine was a strong runner previously.

Comment

Depending on where the crack is, the JB Weld idea is a good one. Properly done it will last as long as the engine. If the crack is external and not between cylinders or the heads you stand a good chance here. Often the crack is on the side of the block. May be pushed out a bit even. There is no real pressure here and if that is all a JB Weld repair costs next to nothing and is worth exploring.
Doug

Comment

If you do go with the 454's one other thing to check is the engine room vents. Engines need fresh air and the larger the engine, the more it needs. Often there is just enough vents for the engine installed. No big thing but it could cause a performance issue if there isn't enough.

Comment

OMC made 2 Cobra drives. The Cobra and King Cobra. The King was for the larger engines and because it wasn't produced in great numbers, some parts are hard to find and are expensive. The regular Cobra drive is easy to maintain. I question the ability of it to withstand the torque of the larger engines. If it could, there would have been no reason to have the King.
If you do change, cautious use of the throttle getting up to speed would be my suggestion.
Doug

Comment

Small block Chevy 5.7 & Ford 5.8 were the biggest engine OMC mated to a regular Cobra drive.
Earlier big block 460 was coupled to a dog clutched King Cobra AKA baby king,
later on coupled to a cone clutched hump back King Cobra... must be a good reason for them to doing it!

Comment

Thank you all for the great responses and huge amount of intellect here!

First, I want to note that my desire in looking at the 454's was due to their local availability only, not based on an upgrade in power. NOT that that's a bad thing in any way because, lets be honest..... ITS MORE POWER!!!! But in all honesty, they are at a great price and it would be a perfect opportunity to upgrade what is currently on the boat. Having said that though, I believe the overall aspects ($$$) of what may be required to fully take on this type of swap significantly outweigh my wants/needs and wifes desire that I not sink a ton of $$$$$$ into said boat! HA!

Back on the hunt for a 5.7 L OMC!!!!!

As always, my sincere thanks to all for the added advice and continued guidance!!!!